1. Field of the Invention
The present invention relates to laser shock processing, and more specifically, it relates to techniques for contouring metal by laser peening.
2. Description of Related Art
Using high power lasers to improve material properties is one of the most important industrial applications of lasers. Lasers can transmit controllable beams of high-energy radiation for metalworking. Primarily, the laser can generate a high power density that is localized and controllable over a small area. This allows for cost effective and efficient energy utilization, minimizes distortions in surrounding areas, and simplifies material handling. Since the laser pulse involves the application of high power in short time intervals, the process is adaptable to high-speed manufacturing. The fact that the beam can be controlled allows parts having complex shapes to be processed. Also accuracy, consistency, and repeatability are inherent to the system.
Improving the strength of metals by cold working undoubtedly was discovered early in civilization, as ancient man hammered out his weapons and tools. Since the 1950s, shot peening has been used as a means to improve the fatigue properties of metals. Another method of shock processing involves the use of high explosive materials in contact with the metal surface.
The use of high intensity laser outputs for the generation of mechanical shock waves to treat the surfaces of metals has been well known since the 1970s. The laser shock process can be used to generate compressive stresses in the metal surfaces adding strength and resistance to corrosive failure.
Lasers with pulse outputs of 10 to 100 J and pulse durations of 10 to 100 ns are useful for generating inertially confined plasmas on the surfaces of metals. These plasmas create pressures in the range of 10,000 to 100,000 atmospheres and the resulting shock pressure can exceed the elastic limit of the metal and thus compressively stress a surface layer as deep or deeper than 1 mm in the metals. Lasers are now becoming available with average power output meaningful for use of the technique at a rate appropriate for industrial production.
In the process of laser shock processing, a metal surface to be treated is painted or otherwise made xe2x80x9cblackxe2x80x9d that is, highly absorbing of the laser light. The black layer both acts as an absorber of the laser energy and protects the surface of the part from laser ablation and from melting due to the high temperature of the plasma. A thin layer of water, typically 1 to 2 mm, is flowed over this black surface. The water acts to inertially confine or, as it is called, tamp the plasma generated as the laser energy is absorbed in the short time pulse duration, typically 30 ns. Other suitable materials that act as a tamper are also possible. A limitation to the usefulness of the process is the ability to deliver the laser energy to the metal surface in a spatially uniform beam. If not uniform, the highest intensity area of the light can cause a breakdown in the water which blocks delivery of meaningful energy to the painted metal surface. A conventional technique to deliver the laser light to the surface is to use a simple lens to condense the laser output to a power density of roughly 100 J to 200 J per square centimeter. This condensing technique has the limitation that a true xe2x80x9cimagexe2x80x9d of the laser near-field intensity profile is not obtained at the surface. Rather a field intensity representing something between the near and far fields is generated. Diffraction of the laser beam as it is focused down onto the surface results in very strong spatial modulation and hot spots.
Any phase aberrations generated within the beam, especially those associated with operation of the laser for high average power, can propagate to generate higher intensity areas within the beam. These high peak intensity regions cause breakdown in the water layer, preventing efficient delivery of the laser energy to the surface to be treated. Another potential cause of breakdown in the tamping material is the generation of non-linear effects such as optical breakdown and stimulated scattering. In a normal generation of a 10 ns to 100 ns pulse within a laser, the output slowly builds over a time period exceeding several pulsewidths. This slow, weak intensity helps to seed the non-linear processes that require buildup times of 10s of nanoseconds. In conventional techniques, the pulse output of the laser is xe2x80x9cslicedxe2x80x9d by an external means such as a fast rising electro-optical switch or by an exploding foil. These techniques can be expensive and can limit reliability.
A controlled application of compressive stress applied to one side of a metal surface will cause that surface to expand in a predictable manner and can thus curve the metal in a highly controllable fashion. Upon curving, the convex surface is left with a residual compressive stress which is highly desirable for fatigue and corrosion resistance of the part in operation. The technique of inducing this compressive stress by means of shot peening is well known and in general use. However, shot peening is limited in the depth of intense compressive stress that can be induced without generating significant and undesirable cold working of the surface layer. Due to the required spherical shape of shot used for peening, the process imparts a non-uniform pressure vs. time profile to the metal during each individual impact of the shot. Pressure is initiated at the first contact point of the sphere and then spreads across the impact area as the metals deform and the entire cross-section of the shot contacts the metal. This non-uniform application of pressure results in a local extrusion of the metal, a flow of metal from the center to the outer area of the impact zone. Consequently, more cold work is done on the metal as material extrudes due to the wedge of pressure created by the impact of the shot.
U.S. Pat. No. 4,694,672, titled xe2x80x9cMethod And Apparatus For Imparting A Simple Contour To A Workpiece,xe2x80x9d is directed to a conventional method and apparatus for imparting a simple contour to an aircraft skin. A treatment chamber has a conveyor with a workpiece attached thereto and includes a shot peening blast unit for treatment of the workpiece. A control system is provided for orienting the workpiece and blast unit so that the peening is done only in narrow spanwise strips and only on common chord percentage lines of the workpiece. This method and apparatus thereby creates chordwise simple curvature to the workpiece while minimizing compound curvature effects. See also U.S. Pat. No. 3,668,912.
In U.S. Pat. No. 4,329,862, titled xe2x80x9cShot Peen Forming Of Compound Contours,xe2x80x9d a flat sheet metal part is conventionally shot peened on both sides. The part is shot peened on one side with an intensity programmed to vary in a pattern for matching the part to a chordwise curvature, and the part takes on the compound curvature of an aircraft wing surface.
It would be desirable if a laser process could achieve an intense stress much deeper into the part and thus effect greater curvature of thicker parts. It would additionally be desirable if the laser process could generate negligible cold work and thus leave a very smooth surface finish.
It is an object of the present invention to provide a laser peening process that can achieve an intense stress much deeper into the part and thus effect greater curvature of thicker metal sections.
The invention is a method and apparatus for forming shapes and contours in metal sections by generating laser induced compressive stress on the surface of the metal workpiece. The laser process can generate deep compressive stresses to shape even thick components without inducing unwanted tensile stress at the metal surface. The precision of the laser-induced stress enables exact prediction and subsequent contouring of parts.
In the present invention, a light beam of 10 to 100 J/pulse is imaged to create an energy fluence of 60 to 200 J/cm2 on an absorptive layer applied over a metal surface. Typically, water is flowed over the absorptive layer. The absorption of laser light causes a plasma to form and consequently creates a shock wave that induces a deep residual compressive stress into the metal. The metal responds to this residual stress by bending.
It is a well-known concept to use a mechanical means to contour thin metal components. The concept of using a laser to generate shock waves that induce a compressive stress into metals is widely practiced for improving the resistance of metal components to fatigue cracking and corrosion. The present invention applies a laser-induced shock to one side of a metal workpiece to produce a precise local curvature. By applying the shock more generally over a broader area, or multiple times over the same area, larger scale curvatures are achieved. This invention employs a high energy, high average power laser, set to operate at specific parameters to achieve precise shaping of components. This laser peen forming process is especially useful for thick (greater than xc2xe inch thick) material that is difficult to shape or contour.
The metal is covered with a layer of material that absorbs the laser light. A thin layer of water is flowed over the absorptive material and illuminated by the laser. By sequentially applying laser pulses in a raster scan fashion, compressive stress is induced over the illuminated surface. The stress will in turn generate a strain of the top layer of metal and produce a curvature to the material. The intensity and depth of compressive stress applied to each local area can be controlled by selecting the laser energy, laser pulse footprint and overlap, the pulse duration and the number of pulses applied to each area. The part can be precisely contoured over its larger area by systematically applying impulses of local stress thereover. Additional control of two-dimensional curvature can be achieved by the specific density in which pulses are placed on the surface that becomes convex, by placing compensating pulses on the surface that becomes concave and by taking advantage of the increasing mechanical moment of inertia generated within the part as a component changes to a curved shape.
The laser peening technique can also be used to precisely straighten components that have an undesired curvature. An important example includes mechanical drive shafts that can acquire an unwanted bend as a result of machining, heat treatment, hardening or other manufacturing processes. By selectively applying compressive stress to the concave side of the unwanted curvature, a part can be systematically straightened.
In contrast to shot peening, the highly uniform laser intensity profile (when using a tailored beam with flat top profile and imaging this profile onto the part) impacts the metal uniformly over the entire impact area resulting in a xe2x80x9cbluntxe2x80x9d force that causes little extrusion of metal and little cold work. Thus the laser peen forming process can produce a greater volume of stressed metal with little cold work. Relatively large curvatures can be impressed into thick metal sections without severely distorting the metal surface.